Honestly, I go back and forth on this issue. I have cooked good food with the water pan (won second place in chicken catagory using it) and have cooked good food with out it. I am currently not using it, but I may go back. My only issue with it is that in the 18.5" that I have the water pan is deep and doesn't offer much clearance to the fire for long cooks.
The one take away I have is, when using ATC like my BBQ Guru DidgiQ, I think I like no water better. When going without the ATC, I think I like water better.
My advice is pick one way, get comfortable cooking with that method, produce good food with that method, then try the other method. If you are proficient at cooking both ways, than just pick the method you like most.